Hi there,
According to P3 log, 15seconds loading for just the WP Core. The theme and plugins are just about 0.1 to 0.6 second. Which makes my observation that your site’s load about 15 to 20 seconds.
There are many forms of optimization, maybe your current one isn’t the proper one on your current site’s host. Example, since your WP Core is slow, maybe memory caching (Memcached) is much acceptable.
Are you on shared hosting? I just noticed that memory usage always stays on 50mb to 64mb, I’m not saying it’s should be greater. I’ just wondering if your hosting has memory limit that can’t be overridden through WordPress configuration. And it’s mostly true on shared hosting.
Thanks!